1. Field of the Invention
The invention relates to a self-regulating heating element for heating a gaseous flowing medium.
2. Description of Related Art
A heating element of the type to which this invention is directed exhibits a strong relationship between the temperature and the electrical resistivity and is used, for example, for electrical heating furnaces and electric driers, and especially, for intake air preheating in internal combustion engines. To preheat the intake air in internal combustion engines, the heating element is installed in a so-called heating flange in the intake duct of the internal combustion engine.
The known self-regulating heating elements for heating of flowing gaseous media of published European Patent Application No. 0 616 486 A1 uses barium titanate ceramics (BaTiO.sub.3) with varied doping as the heating source. In a temperature window from 30 to 40.degree. K, these ceramics exhibit a steep rise of electrical resistivity. Depending on the doping, this range can be, for example, between 220 and 250.degree. C. Thus, ceramic temperatures up to a maximum 250.degree. C. can be attained. The heat is released to the gaseous media by heat transfer members, for example, formed of metal strips bent back and forth in a zig-zag or meander manner, which are in contact with the barium titanate ceramic heating elements and which then have a surface temperature of 80 to 120.degree. C. However, at high flow velocities, for example, in intake air preheating in internal combustion engines, for better heat transfer, a larger temperature difference between the heating element surface and the gaseous medium is necessary. Moreover, Barium titanate ceramics have the disadvantage that they are costly.
Non-selfregulating heating elements for heating of flowing gaseous media are also known in which a metal with relatively high electrical resistivity and good resistance to scale, for example, a CrFeAl alloy, is used. These heating elements reach high temperatures, but have very long heat-up times. This also applies to heating elements which are not externally controlled. However, external controls, for example, in the form of pulse width modulation, are associated with relatively high costs.
The problem of building self-regulating heating elements consists, moreover, in that the heat conductors with high control or regulation properties are extremely susceptible to corrosion.